1. Field of the invention
The present invention relates to a process for the preparation of olefin polymers. In particular, it relates to a process for the preparation of ethylene homo- and copolymers having very high molecular weights and narrow molecular weight distributions.
2. Description of the prior art
It is known that the properties of ethylene homopolymers and of copolymers of ethylene with one or more alpha-olefin comonomers depend on their molecular weight. The use of these polymers in fields such as, for example, the production of films, generally requires that they be endowed with high molecular weights.
The above mentioned ethylene polymers are generally prepared in the presence of catalysts of Ziegler-Natta type. The polymer thus obtained have a wide molecular weight distribution.
More recently, in processes for the preparation of polymers of ethylene, homogeneous catalysts based on metallocene compounds have been used. The polymers obtained by working in the presence of these catalysts, have a narrow molecular weight distribution, but their molecular weights are not always industrially acceptable.
Only when working under particular conditions and in the presence of particular metallocene catalysts such as, for example, metallocene compounds of hafnium, it is possible to obtain ethylene polymers having higher molecular weights.
In the international patent application WO 91/02012, for example, there is described a process for the preparation of high density linear polyethylene having high molecular weight and narrow molecular weight distribution, in the presence of bis(cyclopentadienyl)hafnium compounds and a tetrakis(pentafluorophenyl)borate salt. However, in this application only the preparation of ethylene homopolymers is exemplified.
Metallocene compounds wherein the ligands consist of two substituted cyclopentadienyls linked between them by a bridging group which gives steric rigidity to the molecule, are known to be stereospecific catalyst components for the preparation of isotactic polyolefins. These metallocenes can exist in two sterical configurations, that is the racemic and the meso isomeric form. As the chiral racemic form only is stereospecific, the meso form is generally removed by separation from the rac/meso mixtures obtained from the metallocene synthesis.